The suspected, an 18-year-old man, was arrested in the port of Dover on the English Channel by Kent police.

Basu cautioned the investigation was ongoing and the terrorist threat level remains at "critical," meaning British security services believe that an attack is imminent.

Basu's comments imply other suspects may still be at large.

Police Commissioner Cressida Dick said the arrest was "very significant" but warned the public should still be vigilant and report suspicious activity to authorities.

At 08:20 BST this morning, an improvised explosive device (IED) detonated onboard a passenger car in the London Underground, Parsons Green station.

The blast consumed a section of the the carriage and 29 passengers suffered injuries. Experts stated the device did not detonate properly and if it had, massive loss of life would have occurred.

The IED took the innocuous form of a white plastic bucket inside a supermarket shopping bag.

Shortly after news of the attack broke, the Islamic State claimed responsibility on its Amaq propaganda outlet, claiming that an affiliated Islamist cell within the U.K. orchestrated the bombing.

Hours after the bombing was confirmed as a terrorist incident, officials elevated the terror threat level to 'critical,' the highest possible category, though no secondary attack has been reported.

A massive manhunt is underway with Metropolitan Police augmented by British soldiers. In a statement, the Metropolitan Police force said they "are making fast-time inquiries to establish who was responsible and are working closely with the security services."

MI5 has launched an investigation into the attack, along with hundreds of detectives who are employing forensics, scanning surveillance camera footage, and interviewing witnesses.

No arrests have been made and Scotland Yard has not released any information on possible suspects.

In a statement, London Mayor Sadiq Khan said, “Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,”

Adding, "As London has proven again and again, we will never be intimidated or defeated by terrorism."

Khan was come under criticism for his lax approach on the growing issue of Islamism in the U.K., as the country struggles to keep its citizens safe, with Islamist attacks becoming part and parcel of London—a city that is home to 100 official sharia courts.

In light of of the Charlie Hebdo shootings which claimed the lives of multiple cartoonists, head of MI6, Sir John Sawers, recommended self-censorship and "some restraint" when discussing Islam. Islamists raided the offices of the French satire magazine and murdered staff in retaliation for their caracitures of the Prophet—the outlet no longer draws caracitures of Mohammed.

Simon Collis, British ambassador in Saudi Arabia, converted to Islam and completed the pilgrimage to Mecca, the hajj. He now calls himself Haji Collis.